Dentures, obturators and other prosthetic dental devices typically are made from acrylic resins, such as methyl methacrylate polymers or copolymers. The acrylic resin is generally processed in brass denture flasks for compression molding of the acrylic resin into the desired configuration of the prosthetic dental device while the acrylic resin is in a putty or dough-like stage. Denture flasks are known in the prior art, e.g., U.S. Pat. Nos. 28,688; 61,174; 283,487; 715,182; 1,347,205; 1,500,155; 1,647,048; 1,862,699; 2,102,266; 3,988,094; and 4,218,205.
One of the most successful brass dental flasks has been the three section, ejector type flask marketed by the Hannau Division of Teledyne Dental which is a division of Teledyne Inc. under the trademark of VARSITY. This and similar brass flasks are typically placed in a temperature-controlled water bath for a specified time (usually a 160.degree. F. water bath for about 8 hours in the "long" cure procedure or a 160.degree. F. water bath for about 11/2 hours followed by a 212.degree. F. water bath for about 1/2 hour in the "short" cure procedure) to permit polymerization of the monomer to the polymer to occur. This polymerization is activated by the conduction of the ambient external heat through the brass dental flask.
In 1968, Hasimoto et al. in J. Jap. Res. Soc. Dent. Mater. Appli. 17:46 (1968) reported the use of microwave irradiation to activate resin. In 1983, Kimura et al. J. Osaka Univ. Dent. Sch. 23:43-49 (1983) described the use of microwave irradiation to polymerize and dough form denture base acrylic resin, the disclosure of which is incorporated herein by reference. The use of microwave irradiation in processing acrylic and other denture base resins permits the polymerization/processing time to be significantly reduced to times of less that 60 minutes, and times as short as 30 to 40 minutes. This shortened processing time available with microwave irradiation permits a dentist to complete the fabrication of dentures, obturators and other dental prosthetic devices in several hours, thereby improving his efficiency in treating patients and permitting him to see more patients in a day. The improved efficiency and increase in number of patients seen translates directly into increased revenues for the dentist.
However, polymerization of acrylic and other dental resins using microwave irradiation cannot be done in the traditional brass or other metal denture flasks which reflect the microwaves during processing. Plastic denture flasks, including fiber-reinforced plastic denture flasks, are known for use in microwave irradiation processing and are marketed by the U.S. Shizai Corp. of Santa Monica, Calif. and the H. D. Justi Company of Oxnard, Calif. These flasks are, however, deficient in a number of respects that results from the fiber-reinforced plastic composition used to manufacture these flasks. These deficiencies include and are not limited to lower compression strength (less than 1500 psi) and expansion of the polycarbonate connection bolts upon heating in the microwave oven. A need, therefore, exists for a plastic dental flask of an improved composition having an improved compression strength and which is suitable for use in microwave irradiation processing of dentures, obturators and other prosthetic dental devices.